I thought LLMs had been good at this for a while? I vibed a language out and the LLM could write in it pretty well. I even got Qwen 3.8 to write it (though it was pretty slow, it managed to get it done)

Yeah GPT and Claude both write my language really well[1]. GLM and Kimi did OK as well, but I haven't tried Qwen.

[1] https://march-lang.org/